Plant gain calibration for an optical storage servo system

ABSTRACT

A gain calibration method for optical storage servo systems in which, plant gain calibration is used by injecting a reference sine wave r into an optical storage servo system, obtaining an effort signal m at the input of the servo plant and an error signal y at the output of the servo plant, using a DFT (Discrete Fourier Transformation) to translate the time domain signals m and y into frequency responses M and Y, calculating a Y-to-M ratio, and using the magnitude of the Y-to-M ratio as the plant gain K of the servo system. The servo system&#39;s sensor gain K1 at the outermost layer of a disk may be calibrated by, e.g., the conventional peak-to-peak measurement. Since K=K1·K2, the servo system&#39;s actuator gain K2 at the outermost layer of the disk may be obtained. Because the actuator gain K2 is the same for all layers of a disk, the variation of the sensor gain K1 at an inner layer may follow that of the plant gain K at that layer. Accordingly, the sensor gain K1 may be calibrated for each layer of a multilayer disk, and the calibration is independent from the compensator C.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to previously filed U.S.provisional patent application Ser. No. 60/955,935, filed Aug. 15, 2007,entitled Method of Plant Gain Calibration for Optical Storage ServoSystem. That provisional application is hereby incorporated by referencein its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to optical storage systems, andmore particularly to a gain calibration method for an optical storageservo system.

2. Description of Related Art

An optical storage servo system is used to control the positioning of anoptical lens, so that a track on an optical disk will be positionedaccurately beneath the lens. In contrast to a hard disk drive, anoptical storage system uses removable media. As a result, both a loaderfor the disk, and the disk itself, may introduce disturbances duringoperation of the optical storage system. An optical storage servo systemmay have several feedback control loops, such as a focus loop and aradial loop, to achieve high performance and robustness against OPU(optical pickup unit) actuator variations and other mechanicaldisturbances coming from disks or an actuating mechanism.

FIG. 1 illustrates an example of a simplified servo feedback loop for anoptical storage system. As shown, the loop may have a dynamic plant Pand a compensator C. The plant P may have an actuator P₁ and a sensorP₂. As a result:P=P ₁ ·P ₂  (1)

The actuator P₁ may include a digital/analog converter (D/A), a motordriver circuit, and a VCM (Voice Coil Motor) in an OPU/loader. Sinceoptical storage systems use low cost motors, each loader may have adifferent actuator gain K₂ and may introduce disturbances oruncertainty. The sensor P₂ may include an optical photo diode, and aservo signal generator module in firmware. The sensor P₂ may detect anerror signal between the OPU's actual position and a target position,and may have different gain K₁ and introduce plant uncertainties. Thesensor gain K₁ may include a focus sensor gain and a radial sensor gain.The compensator (or controller) C may receive the output of the sensor,as a servo signal, and generate a control effort to drive the actuatorand suppress the disturbances or uncertainty. The compensator C may be acontrol IIR (Infinite Impulse Response) filter implemented in firmware.The open loop transfer function of the servo feedback loop shown in FIG.1 is as follows:L=C·K ₁ ·K ₂ ·P ₁ ·P ₂  (2)where K₁ represents a sensor gain, K₂ represents an actuator gain, Crepresents a compensator, P₁ represents the actuator, and P₂ representsthe sensor.

The gain calibration for an optical storage servo system may includecalibration of the actuator gain K₂ and the sensor gain K₁. Since thereare big gain variations in inexpensive optical storage systems, it isvery difficult to accurately measure the actuator gain K₂ in open loopsettings. Currently available technologies use Loop Gain Calibration(LGC) in closed loop operation to calibrate the actuator gain K₂ bymeasuring either the magnitude or the phase of the transfer function(2), as shown in FIGS. 2A and 2B.

The curve for magnitude measurement is shown in FIG. 2A. A fixedfrequency sine wave may be injected into the servo feedback loop shownin FIG. 1 at point A as a reference signal. The frequency of the sinewave may be the zero dB cross-over frequency determined by thecompensator C. The magnitude of the transfer function (2) may bemeasured at point B. Since the target is zero dB or 1, the actuator gainK₂ is the inverse of the loop magnitude measurement at the cross-overfrequency.

The curve for phase measurement is shown in FIG. 2B. A fixed frequency,fixed magnitude sine wave may be injected into the servo feedback loopshown in FIG. 1 at point A as a reference signal, and the phasedifference between the servo error signal at point B and the injectedsine wave may be obtained. The sine wave may be, for example, 1.36 kHz.The phase difference may be compared with a target. A trial-and-errormethod may be used to make the phase difference to approach the target,so as to search for the actuator gain K₂. One problem of the phasedetection based LGC is that it suffers from defective disks, since it isdifficult to implement a defect protection scheme when using thetry-and-error method.

Currently available technologies use an open loop sensor peak-to-peakmeasurement to calibrate the sensor gain K₁. In focus sensor gaincalibration, an open loop focus ramp may be performed so that thepeak-to-peak value of a focus error S-curve signal may be measured andcompared with a target value. The S-curve peak-to-peak value is shown inFIG. 3 as the “pFocusError.” Similarly, the radial sensor gain may becalibrated by measuring the peak-to-peak value of a radial error signalin a radial open loop. One problem is that the method can only be usedwith single layer disk servo systems, e.g., CDs or single layer DVDs.However, multilayer format optical systems, e.g., red laser DVDs, bluelaser BDs and HD-DVDs, are becoming popular, and the focus sensor gainK₁ at inner layers may have a different S-curve than that of theoutermost layer. Accordingly, the sensor gain K₁ cannot be accuratelycalibrated at inner layers with the currently available technology.

In addition, the currently available technologies assume that thecompensator C is fixed, and therefore include the compensator C in thepath from point A to B and calibrate the loop gain of the optical servosystem. The currently available technologies also assume that thevariation of the actuator gain K₂ is the same as that of the transferfunction (2) since the compensator C is constant and the sensor gain K₁can be calibrated very accurately by the open loop sensor peak-to-peakmeasurement. However, in some optical storage servo systems, the spindlespeed may depend on disk conditions, and a particular compensator C maybe provided for each spindle speed. When the frequency of the injectedsine wave stays the same, if C changes, the zero crossing point may moveand may affect the calibration of the actuator gain K₂.

Therefore, it may be desirable to provide a method to improve thecalibration of the optical storage servo system.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

Embodiments of the present invention are described herein with referenceto the accompanying drawings, similar reference numbers being used toindicate functionally similar elements.

FIG. 1 illustrates an example of a simplified servo feedback loop for anoptical storage system.

FIG. 2A illustrates an exemplary curve of magnitude measurementsobtained by using LGC.

FIG. 2B illustrates an exemplary curve of phase measurements obtained byusing LGC.

FIG. 3 illustrates an exemplary S-curve obtained in an open loop sensorpeak-to-peak measurement for calibrating a sensor gain.

FIG. 4 illustrates signals used in plant gain calibration (PGC) for anoptical storage servo system according to one embodiment of the presentinvention.

FIG. 5 is a flow chart of a method of plant gain calibration for anoptical storage servo system according to one embodiment of the presentinvention.

FIG. 6A illustrates an exemplary frequency response at the input of aservo plant according to one embodiment of the present invention.

FIG. 6B illustrates an exemplary frequency response at the output of aservo plant according to one embodiment of the present invention.

FIG. 7 illustrates exemplary waveforms for defect detection according toone embodiment of the present invention.

FIG. 8 is a flow chart of a method for controlling transfer function ofan optical servo system according to one embodiment of the presentinvention.

FIG. 9 illustrates a block diagram of an apparatus for controllingtransfer function of an optical storage servo system according to oneembodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a gain calibration method for opticalstorage servo systems. Instead of loop gain calibration, the method ofthe present invention uses plant gain calibration by injecting areference sine wave r into an optical storage servo system, obtaining aneffort signal m at the input of the servo plant and an error signal y atthe output of the servo plant, using a DFT (Discrete Fourier Transform)to translate the time domain signals m and y into frequency responses Mand Y, calculating a Y-to-M ratio, and using the magnitude of the Y-to-Mratio as the plant gain K of the servo system. The servo system's sensorgain K₁ at the outermost layer of a disk may be calibrated by, e.g., theconventional peak-to-peak measurement. Since K=K₁·K₂, the servo system'sactuator gain K₂ at the outermost layer of the disk may be obtained.Because the actuator gain K₂ is the same for all layers of a disk, thevariation of the sensor gain K₁ at an inner layer may follow that of theplant gain K at that layer. Accordingly, the sensor gain K₁ may becalibrated for each layer of a multilayer disk, and the calibration isindependent from the compensator C. The method may be used with signallayer optical disks and multilayer optical disks. Advantages of thepresent invention will become apparent from the following detaileddescription.

FIG. 4 illustrates signals used in plant gain calibration for an opticalstorage servo system according to one embodiment of the presentinvention. As shown, a reference sine wave signal r may be injected intothe servo system. A time domain response at the input m of the servoplant may be used as an effort signal, and a time domain response at theoutput y of the servo plant may be used as an error signal. Signals mand y may be used for the PGC of the present application.

FIG. 5 is a flow chart of a method of plant gain calibration for anoptical storage servo system according to one embodiment of the presentinvention. The method may be used in the optical storage servo systemshown in FIG. 4.

Removable media used in optical storage systems may have defects. Adefect in an optical storage servo system means that a servo signal froma disk is damaged and cannot be used for feedback control. The defectmay be, e.g., scratches, dots, or fingerprints. A large defect may be achallenge for the optical storage servo system. In terms of the strengthof light reflected from a disk, e.g., Q_(sum), there are two kinds ofdefects: black dots and bright dots. In the case of black dots, there isno reflection from the disc, and therefore the Q_(sum) signal may drop.In addition, the Q_(sum) signal may have transient at both start and endof defect, which may cause a spike. In FIG. 7, the second defect may bea black dot. In the case of bright dots, the disk surface may havemanufacture problems, such as an air bubble, which may cause reflectionsfrom the disk high enough to distort the servo signal. In FIG. 7, thefirst defect may be a bright dot. A defect flag with a long delay may beused to mute the servo transient effect for a discrete-time Fouriertransformation (DFT) engine. For the second defect in FIG. 7, the servoperformance is not good during the long delay period. If the DFT engineis started, it may bring the PGC accuracy down.

At 510, it may be determined whether Q_(sum) has dropped below apredetermined threshold.

If yes, at 511, a defect flag may be asserted, as shown in FIG. 7. Thedefect flag may be de-asserted (at 512) after a predetermined timedelay, so as to return to normal track following.

Plant gain calibration may occur simultaneously with the defectdetection. At 500, time domain responses to the reference signal r atthe input m of the servo plant and the output y of the servo plant maybe obtained, wherein m is an effort signal, and y is an error signal.

At 501, signals m and y may be translated from the time domain into thefrequency domain and become frequency responses M and Y.

In one embodiment, a Fourier transform (phase and magnitude) of a servoplant at a particular frequency may be measured and the measurement maybe compared with a target value. A continuous Fourier transform may bedefined as follows:V(f)=∫_(−∞) ^(∞) v(t)e ^(−j2πft) dt  (3)

In one embodiment, a DFT may be used to approximate the continuousFourier transform as follows:

$\begin{matrix}{{{V\lbrack k\rbrack} = {\sum\limits_{n = 0}^{N - 1}\;{{v\lbrack k\rbrack}{\mathbb{e}}^{{- j}\; 2\pi\;{{kn}/N}}}}},{0 \leq k \leq {N - 1}}} & (4)\end{matrix}$wherein N is the number of samples.

The accuracy of the approximation may depend on the number of samplesand the sampling frequency. In one embodiment, 96 sine waves at 1.36 kHzmay be injected into the servo plant. By measuring the DFT of the servoplant output y and the DFT of the servo plant input m, the continuousFourier transform of the servo plant (magnitude and phase) may beapproximated. FIG. 6A illustrates an example of the reference signal rand its frequency response M, which is a radial effort signal. FIG. 6Billustrates an example of the reference signal r and its the frequencyresponse Y, which is a radial error signal.

When the SNR (Signal-to-noise Ratio) is above a target level, one DFTsession may be enough. One DFT session may be defined as one period ofinjected sine wave shown in FIGS. 6A and 6B. However, in order toovercome the large disturbances and noise in the closed loop system, PGCmay need to have many DFT sessions to obtain a reliable and accurateplant gain result.

In the example shown in FIGS. 6A and 6B, one DFT cycle of the radialeffort signal may include the following 64 values:

7 −30 −8 −19 −30 −22 −2 −15 −44 55 −35 14 13 −15 −27 −7 −55 −80 −67 −6118 29 59 24 −8 −9 −33 −39 −66 −58 −22 15 4 12 54 −47 −18 −12 33 −21 29 9−1 12 −4 15 3 −15 5 66 −35 −35 −18 37 −49 −5 −1 46 28 33 46 −46 55 −67

Thus, the frequency response M may be:M=(45.4)+j(394.2)

In the example shown in FIGS. 6A and 6B, one DFT cycle of radial errorsignal may include the following 64 values:

−33 −13 −7 5 21 31 34 43 61 38 61 53 52 60 70 69 88 109 121 130 109 9165 55 55 50 54 57 69 73 63 42 31 16 −14 1 −8 −15 −38 −34 −55 −62 −67 −76−78 −87 −90 −85 −90 −114 −92 −81 −77 −90 −65 −65 −63 −76 −76 −79 −86 −55−73 −32

Consequently, the frequency response Y may be:Y=(−790.4)+j(−2845)

Before each DFT session, the defect flag may be checked at 502. If thedefect flag is asserted, the procedure may go to the next DFT session,so as to discard signals from defect areas and avoid using signals fromdefect areas in calculating the plant gain.

At 503, a Y-to-M ratio, which is the plant transfer function L, may bedetermined. In one example, the plant transfer function L may be:

$L = {\frac{Y}{M} = \frac{\left( {- 790.4} \right) + {j\left( {- 2845} \right)}}{(45.4) + {j(394.2)}}}$

At 504, the magnitude of the plant transfer function L, which is thegain K of the dynamic servo plant at the outermost layer may becalculated. In one example, the magnitude may be:

$K = {{\frac{Y}{M}} = {{\frac{\left( {- 790.4} \right) + {j\left( {- 2845} \right)}}{(45.4) + {j(394.2)}}} = {\frac{\sqrt{\left( {- 790.4} \right)^{2} + \left( {- 2845} \right)^{2}}}{\sqrt{(45.4)^{2} + (394.2)^{2}}} = {\frac{2952.6}{396.8} = 7.44}}}}$

At 505, the servo plant's sensor gain K₁ at the outermost layer of thedisk may be calibrated, e.g., by the conventional peak-to-peakmeasurement.

As shown in the equation (1), the gain of the servo plant includes theactuator gain K₂ and the sensor gain K₁. When the servo plant's sensorgain K₁ at the outermost layer and the plant gain K are obtained at 505and 504 respectively, the servo plant's actuator gain K₂ at theoutermost layer of the disk may be determined at 506.

Since the actuator gain K₂ is the same for all layers of a disk, thevariation of the sensor gain K₁ of an inner layer may follow that of theplant gain K at that layer. If it is determined at 507 that the disk hasanother layer, 500-504 may be performed at 508 to determine the servoplant's gain K at the second layer, and the sensor gain K₁ at the secondlayer may be calibrated at 509. 507-509 may be repeated for each layerof a multilayer disk.

Since signals at the input and output of the servo plant are used duringthe calibration, the PGC is independent of the compensator C. Inaddition, it is easier to implement a robust defect protection schemeduring the PGC, so that the PGC result is accurate even for disks withvarious kinds of defects.

FIG. 8 is a flow chart of a method for controlling the transfer functionof an optical storage servo system according to one embodiment of thepresent invention. The method shown in FIG. 8 may be used in the opticalstorage servo system shown in FIG. 4, and may use part of the methodshown in FIG. 5 to calibrate the plant gain of an optical storage servosystem.

At 801, a disk may be loaded into an optical storage system.

At 802, the plant gain K at the outermost layer of the disk may becalibrated through 501 to 504.

At 803, it may be determined whether the plant gain K at the outermostlayer equals one of a number of nominal plant gains K_(nn) stored in amemory. The memory may also store a compensator C_(nn) for each nominalplant gain K_(nn). In one embodiment, a pair of nominal plant gainK_(nn) and its corresponding compensator C_(nn) may be stored for afamily of disks, e.g., CD, DVD, and Blu-ray DVD, so that a compensatorC_(nn) may be used for the whole family of disks. A compensator C_(nn)designed with a nominal plant gain K_(nn) may deliver the same closedloop performance on different drives, even though there may besignificant actuator and sensor variations.

If the plant gain K at the outermost layer equals a nominal plant gainK_(n1), at 804, the compensator C_(n1) corresponding to the nominalplant gain K_(n1) may be selected as the compensator for the opticalstorage servo system, and the procedure may proceed to 807.

If the plant gain K at the outermost layer is different from all thestored nominal plant gains K_(nn), the plant gain K at the outermostlayer may be adjusted to a closest nominal gain, e.g., K_(n2), at 805,and the compensator C_(n2) corresponding to the closest nominal gainK_(n2) may be selected for the optical storage servo system at 806 Theprocedure then may proceed to 807.

At 807, the sensor gain K₁ at the outermost layer of the disk may becalibrated by, e.g., the conventional peak-to-peak measurement. In oneembodiment, a compensator C_(n0), which may tolerate high disturbancesand noise, may be used for the calibration.

At 808, the actuator gain K₂ at the outermost layer may be calculated.

At 809, it may be determined whether the disk has another layer. If not,the procedure may end at 899.

It the disk has a second layer, then at 810, the plant gain K at thesecond layer may be calculated through 501-504, and at 811, it may bedetermined whether the plant gain K at the second layer is the same asthe plant gain at the outermost layer. If yes, C_(n1) which correspondsto the nominal plant gain of the outermost layer K_(n1) may be selectedfor the second layer at 812, and the procedure may return to 809.

If the plant gain K at the second layer is different from that of theoutermost layer, then at 813, the plant gain K at the second layer maybe adjusted to equal the plant gain K at the outermost layer, so thatthe compensator C_(n1) selected for the outermost layer may be used forthe second layer as well. The procedure may then return to 809.

In one embodiment, at 813, instead of adjusting the plant gain K at thesecond layer, a compensator C_(nn) corresponding to a nominal plant gainwhich equals or is the closest to the plant gain at the second layer maybe selected for the second layer.

Since the compensator C may be selected according to the plant gain ofthe optical storage servo system, the performance of the optical storageservo system may be significant improved.

FIG. 9 illustrates a block diagram of an apparatus for controllingtransfer function of an optical storage servo system according to oneembodiment of the present invention. The apparatus may include a controlunit 901 and a memory 902. The control unit 901 may control variousprocedures of the optical storage servo system, including the proceduresshown in FIGS. 5 and 8. The memory 902 may store a number of nominalplant gains K_(nn) and a compensator C_(nn) corresponding to eachnominal plant gain. The control unit 901 may access the memory 902during operation of the methods shown in FIGS. 5 and 8.

Several features and aspects of the present invention have beenillustrated and described in detail with reference to particularembodiments by way of example only, and not by way of limitation.Alternative implementations and various modifications to the disclosedembodiments are within the scope and contemplation of the presentdisclosure. Therefore, it is intended that the invention be consideredas limited only by the scope of the appended claims.

1. A method for calibrating gains of an optical storage servo system, wherein the optical storage servo system comprises a plant which includes an actuator and a sensor, the method comprising: receiving a reference signal injected into the optical storage servo system; obtaining at an input of the plant an effort signal; obtaining at an output of the plant an error signal; and determining a plant gain of the servo system based on the effort signal and the error signal, wherein a DFT (Discrete Fourier Transformation) is used to translate the effort signal from a time domain effort signal into a frequency domain effort signal, and translate the error signal from a time domain error signal into a frequency domain error signal.
 2. The method of claim 1, further comprising: determining a transfer function of the plant by calculating a ratio between the frequency domain error signal and the frequency domain effort signal.
 3. The method of claim 2, further comprising: determining the plant gain by calculating a magnitude of the transfer function.
 4. The method of claim 1, further comprising: determining a sensor gain of the optical storage servo system at an outermost layer of an optical disk.
 5. The method of claim 4, further comprising: determining an actuator gain of the optical storage servo system at the outermost layer of the optical disk with the plant gain and the sensor gain at the outermost layer of the optical disk.
 6. The method of claim 5, further comprising: determining the plant gain of the optical storage servo system at a second layer of the optical disk.
 7. The method of claim 6, further comprising: determining a sensor gain of the optical storage servo system at the second layer of the optical disk.
 8. The method of claim 1, wherein the optical storage servo system includes an optical disk, the method further comprising: comparing a reflecting signal from the optical disk with a predetermined threshold, and asserting a defect flag when the reflecting signal drops below the predetermined threshold.
 9. The method of claim 8, further comprising: checking the defect flag before determining the plant gain, and ignoring error signals received when the defect flag is asserted.
 10. The method of claim 8, further comprising: de-asserting the defect flag after a predetermined time delay.
 11. A method for controlling a transfer function of an optical storage servo system, wherein the optical storage servo system includes a compensator and a plant, and the plant includes an actuator and a sensor, the method comprising: storing in a memory at least two nominal plant gains and their corresponding compensators; injecting a reference signal into the optical storage servo system; obtaining at an input of the plant an effort signal, which is a time domain response to the reference signal; obtaining at an output of the plant an error signal, which is a time domain response to the reference signal; determining a plant gain of the optical storage servo system based on the effort signal and the error signal, including using a DFT (Discrete Fourier Transformation) to translate the effort signal from a time domain effort signal into a frequency domain effort signal, and translate the error signal from a time domain error signal into a frequency domain error signal; and selecting a compensator corresponding to the plant gain.
 12. The method of claim 11, wherein the optical storage servo system includes an optical disk, the method further comprising: determining whether the plant gain of the optical storage servo system at an outermost layer of the optical disk is the same as a nominal plant gain.
 13. The method of claim 12, further comprising: when the plant gain of the optical storage servo system at the outermost layer of the optical disk is the same as a nominal plant gain in the memory, selecting the compensator corresponding to the nominal plant gain as the compensator for the servo system.
 14. The method of claim 12, when the plant gain of the optical storage servo system at the outermost layer of the optical disk is different from all nominal plant gains in the memory, adjusting the plant gain of the servo system at the outermost layer of the optical disk to a closest nominal plant gain in the memory.
 15. The method of claim 14, further comprising: selecting the compensator corresponding to the closest nominal plant gain as the compensator for the optical storage servo system.
 16. The method of claim 11, further comprising: determining whether the plant gain of the servo system at a second layer of an optical disk is the same as that at an outermost layer.
 17. The method of claim 16, further comprising: adjusting the plant gain at the second layer to the plant gain at the outermost layer.
 18. The method of claim 17, wherein the plant gain at the second layer is adjusted by changing a sensor gain of the optical storage servo system at the second layer of the optical disk.
 19. The method of claim 17, further comprising: using the compensator selected for the outermost layer as the compensator for the second layer.
 20. The method of claim 11, wherein each pair of a nominal gain and its corresponding compensator are specific to a family of optical disks.
 21. An apparatus for an optical storage servo system, wherein the optical storage servo system includes a compensator and a plant, the apparatus comprising: a memory for storing at least two nominal plant gains and their corresponding compensators; and a control unit which: obtains a time domain effort signal; obtains a time domain error signal; applies a DFT (Discrete Fourier Transformation) to translate the time domain effort signal and the time domain error signal to a frequency domain effort signal and a frequency domain error signal; determines a plant gain based on the frequency domain effort signal and the frequency domain error signal; and selects a compensator corresponding to the plant gain.
 22. The apparatus of claim 21, wherein the control unit further determines whether the plant gain of the optical storage servo system at a first layer of an optical disk is the same as a nominal plant gain.
 23. The apparatus of claim 22, wherein when the plant gain of the optical storage servo system at the first layer of the optical disk is the same as a nominal plant gain in the memory, the control unit selects the compensator corresponding to the nominal plant gain as the compensator for the servo system.
 24. The apparatus of claim 22, wherein when the plant gain of the optical storage servo system at the first layer of the optical disk is different from all nominal plant gains in the memory, the control unit adjusts the plant gain of the servo system at the first layer of the optical disk to a closest nominal plant gain in the memory.
 25. An apparatus for performing the method of claim
 1. 26. The apparatus of claim 25, wherein the reference signal is a sine wave. 